Default Inheritance Protection Levels

In §7.2 (p. 268) we saw that classes defined with the struct and class keywords have different default access specifiers. Similarly, the default derivation specifier depends on which keyword is used to define a derived class. By default, a derived class defined with the class keyword has private inheritance; a derived class defined with struct has public inheritance:

class Base { /* ...   */ };
struct D1 : Base { /* ...   */ };   // public inheritance by default
class D2 : Base { /* ...   */ };    // private inheritance by default

It is a common misconception to think that there are deeper differences between classes defined using the struct keyword and those defined using class. The only differences are the default access specifier for members and the default derivation access specifier. There are no other distinctions.

Image Best Practices

A privately derived class should specify private explicitly rather than rely on the default. Being explicit makes it clear that private inheritance is intended and not an oversight.

Exercises Section 15.5

Exercise 15.18: Given the classes from page 612 and page 613, and assuming each object has the type specified in the comments, determine which of these assignments are legal. Explain why those that are illegal aren’t allowed:

Base *p = &d1;  //  d1 has type Pub_Derv
p = &d2;        //  d2 has type Priv_Derv
p = &d3;        //  d3 has type Prot_Derv
p = &dd1;       //  dd1 has type Derived_from_Public
p = &dd2;       //  dd2 has type Derived_from_Private
p = &dd3;       //  dd3 has type Derived_from_Protected

Exercise 15.19: Assume that each of the classes from page 612 and page 613 has a member function of the form:

void memfcn(Base &b) { b = *this; }

For each class, determine whether this function would be legal.

Exercise 15.20: Write code to test your answers to the previous two exercises.

Exercise 15.21: Choose one of the following general abstractions containing a family of types (or choose one of your own). Organize the types into an inheritance hierarchy:

(a) Graphical file formats (such as gif, tiff, jpeg, bmp)

(b) Geometric primitives (such as box, circle, sphere, cone)

(c) C++ language types (such as class, function, member function)

Exercise 15.22: For the class you chose in the previous exercise, identify some of the likely virtual functions as well as public and protected members.

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